Paracanthopoma, Giltay, 1935

Pinna, Mário de & Dagosta, Fernando Cesar Paiva, 2022, A taxonomic review of the vampire catfish genus Paracanthopoma Giltay, 1935 (Siluriformes, Trichomycteridae), with descriptions of nine new species and a revised diagnosis of the genus, Papéis Avulsos de Zoologia 62, pp. 1-90 : 8-11

Key to species of Paracanthopoma

Although the various species of Paracanthopoma are very different from each other, often by decisive diagnostic traits, perceiving some of those differences can be challenging at first. Their generally white skin lacks contrast, thus rendering superficial details difficult to visualize.What little dark pigment there is, fades rather quickly under normal preservation conditions. Also, the integument is often preserved in folded or deformed positions in critical areas of the body, masking otherwise striking distinctions in shape and proportions. Odontodes and teeth may be deeply sunk in integument, making counts prone to errors. Those difficulties are aggravated in very small specimens and those with a long history of preservation, when normal shrinkage further deforms external morphological traits. Access to cleared and stained specimens, CT and SEM images greatly facilitates identifications, but such resources are not always available.

The key below attempts to circumvent such difficulties to some degree, by placing emphasis on easily observable traits that are less prone to deformation during fixation or long preservation, with less accessible traits (e.g., vertebral number and other osteological data) given as a means of confirmation when necessary. Still, reliance on some meristic data and other minute details is unavoidable in certain cases, and cleared and stained preparations or x-rays are necessary to reliably obtain such information. Procurrent fin rays, teeth and odontodes can rarely be accurately counted on alcoholic specimens of Paracanthopoma , even when well-preserved. Fortunately, the general habitus of the body and, especially, the head of most species are largely diagnostic, provided availability of well-preserved specimens and some degree of familiarity with their range of variation. Although differences of this sort are unappealling when expressed in words, they are fully apprehended visually. Thus, comparative illustrations are offered as aids to identification ( Figs. 3 , 4 ).

1a. Maxillary barbel extending for less than half of distance between its base and anterior limit of interopercular odontodophore;distance between interopercular odontodophore and eye at least 2.5 times distance between two odontodophores;epiphyseal canals joining medially, opening as single median pore on top of skull............................................................................................................................................................................................................................. 2

1b. Maxillary barbel extending for more than half of distance between its base and anterior limit of interopercular odontodophore; distance between opercular and interopercular odontodophore approximately equal or only slightly smaller than that between latter and eye;epiphyseal canals not conjoined medially, opening as two separate pores behind eyes.......................................................................................................................................................................... 5

2a. Origin of anal fin at same vertical through origin of dorsal fin,or only slightly posterior to it;caudal fin truncate or only slightly concave;procurrent caudal-fin rays 19or 20 dorsally and 20 or 21ventrally,the latter series extending anteriorly to vertical through posterior end of anal fin........................... Pc.daemon

2b. Origin of anal fin well posterior to that of dorsal fin(at vertical through middle of base of dorsal fin or further posteriorly);caudal fin clearly bilobed or strongly concave;procurrent caudal-fin rays 14to19 dorsally and 14to18 ventrally,the latter series not reaching anteriorly to vertical through posterior end of anal fin.......................................................................................................................................................................................................................................... 3

3a. Opercular and interopercular odontodophores extremely reduced in size and number of odontodes,inconspicuous on surface of head,opercular odontodophore with one or two odontodes sunk in narrow slit of skin; lateral line strongly angled dorsoposteriorly, its distal pore distant from axillary gland pore, and located posterior to vertical through posterior margin of latter.......................................................................................................................... Pc.truculenta

3b. Opercular and interopercular odontodophores clearly visible on surface of head,with four or more odontodes exposed;lateral line parallel to longitudinal axis of body or only gently curved dorsoposteriorly,its distal opening adjacent to axillary gland pore, and located anterior to vertical through posterior margin of latter ................................................................................................................................................................................................................................. 4

4a. Dorsal surface of skull largely covered with muscle, so that in dorsal aspect the exposed portion of the braincase is equivalent maximally to interorbital; opercular and interopercular odontodophore far smaller than eye...................................................................................................................... Pc.carrapata

4b. Head musculature not covering extensively the dorsal surface of skull,exposed portion of the braincase larger than interorbital;opercular and interopercular odontodophore approximately as large as eye,or only slightly smaller..................................................................................................................... Pc.parva

5a. Median premaxillary teeth 13 or more,forming a rectangular arrangement occupying most or all surface of visible upper jaw.............................................. 6

5b. Median premaxillary teeth 11 or fewer forming a small semicircular or triangular arrangement occupying only middle portion of visible upper jaw............. 7

6a. Odontodophores relatively large, with 11 or 12 opercular and 7 or 8 interopercular odontodes; presence of a bilateral series of irregular dark spots along each side of dorsal midline;vertebrae 40; procurrent caudal-fin rays 19 to 21dorsally and 18 to 20 ventrally;median premaxillary teeth 18 or 19; principal caudal-fin rays 6 + 7 ............................................................................................................................................................................................ Pc.malevola

6b. Odontodophores small,with5 or6 opercular and 4or5 interopercular odontodes;dorsum lacking dark pigment;vertebrae42or 43;procurrent caudal-fin rays 32 dorsally and 30 to 32 ventrally;median premaxillary teeth13;principal caudal-fin rays 6 + 6........................................................................ Pc.satanica

7a. Three pelvic-fin rays; opercular odontodophore minuscule, at middle of large roundish area of thickened integument and periodontodal fold vestigial or absent;prepelvic length 56.0-61.1% SL;caudal peduncle length 24.0-26.6% SL ........................................................................................................ Pc.saci

7b. Five pelvic-fin rays; opercular odontodophore well-developed, surrounded by well-defined, narrow periodontodal fold; prepelvic length 62.1-67.8% SL; caudal peduncle length 17.9-24.0% SL................................................................................................................................................................................. 8

8a. Median premaxilla and associated dentition small,with5or fewer teeth occupying less than half of exposed upper jaw;maxillary barbel short,its tip reaching maximally 75% of distance to interopercular odontodophore;premaxilla lacking any conical teeth;opercular odontodes 6-9............................................ 9

8b. Median premaxilla and associated dentition large, with 11 teeth occupying more than half of exposed upper jaw; maxillary barbel long, its tip reaching interopercular odontodophore or 95% of distance to it; premaxilla with one or two conical teeth near corner of mouth;opercular odontodes 10-13....... 12

9a. Width of opercular periodontodal fold approximately uniform around its perimeter,encircling opercular odontodes only,not extended anteriorly as horizontal ridge of integument;base of maxillary barbel anterior to vertical through anterior margin of eye in lateral view ................................................... Pc.capeta

9b. Ventral portion of opercular periodontodal fold hypertrophied, extending anteriorly in straight line to dorsal margin of interopercular odontodophore, forming prominent horizontal ridge of integument between odontodophores; base of maxillary barbel at or posterior to vertical through eye in lateral view .................................................................................................................................................................................................................................... 10

10a. Intense dark pigment on head, in stark contrast to white body, formed by combination of brain pigment over posterior and lateral parts of neurocranium, plus integumentary pigmentation extending laterally onto area of opercular odontodophore and between opercular and interopercular odontodophores; vertebrae 45; median premaxillary teeth2 to 4 .................................................................................................................................................... Pc.ahriman

10b. Dark pigment on head faint,reduced to few spots of dark brain pigment;vertebrae 39 to 44;median premaxillary teeth 5.................................................. 11

11a. Principal caudal-fin rays 5 + 5; caudal peduncle depth 10.8-13.0% SL; posterior internarial width 8.1-10.0% HL; procurrent caudal-fin rays 28-30 dorsally and 27-29 ventrally;deepest portion of the caudal peduncle (corresponding to longest procurrent caudal-fin rays) approximately at its half-length;dorsal and ventral profiles of caudal peduncle posteriorly strongly converging towards base of caudal fin,forming pronounced concave regions clearly delimiting beginning of caudal fin; interorbital larger than eye diameter ............................................................................................................................. Pc.cangussu

11b. Principal caudal-fin rays 6 + 6; caudal peduncle depth 8.3-10.6% SL;posterior internarial width 3.3-5.5% HL;procurrent caudal-fin rays 19-25 dorsally and 21-25 ventrally; caudal peduncle progressively deeper to base of caudal fin; dorsal and ventral profiles of caudal peduncle gently continuous with caudal fin,with only slight depression in some specimens;interorbital smaller than eye diameter................................................................................... Pc.irritans

12a. Interorbital smaller than eye diameter;caudal fin gently convex or truncate;interopercular odontodes8 or9;vertebrae 40to42;procurrent caudal-fin rays 22 to 27 dorsally and 21to 25 ventrally;caudal-fin rays 5 + 6 or 6 + 6...................................................................................................................... Pc.vampyra

12b. Interorbital equal or larger than eye diameter;caudal fin gently concave or bilobed;interopercular odontodes12-14;vertebrae38or 39;procurrent caudal-fin rays 14 to 19 dorsally and ventrally;caudal-fin rays 6 + 7...................................................................................................................................... Pc.alleynei

Figures

Figure 3. Schematic representations of the head of species of Paracanthopoma in dorsal view. (A) Pc. ahriman; (B) Pc. alleynei; (C) Pc. cangussu; (D) Pc. capeta; (E) Pc.carrapata; (F) Pc.daemon; (G) Pc.irritans; (H) Pc.malevola; (I) Pc.parva; (J)Pc.saci; (K) Pc.satanica; (L) Pc.truculenta; (M) Pc.vampyra.

Figure 4. Comparative chart of jaws and anterior portion of head of Paracanthopoma species, cleared and stained specimens, ventral views: (A) Pc. ahriman FMNH 105525;(B)Pc.alleynei MZUSP 103052;(C) Pc.cangussu MZUSP 86250;(D)Pc.capeta MZUSP 29154;(E) Pc.daemon MZUSP 95597;(F)Pc.irritans INPA 20529; (G) Pc.malevola MCP 36217;(H) Pc.parva MZUSP 30400;(I) Pc.saci MZUSP 125626; (J) Pc.satanica MZUSP 100149;(K) Pc.truculenta MZUSP 30404;(L) Pc.vampyra MZUSP 100138. Scale bars = 500 μm. Pc.carrapata not shown.